Due to relatively high instances of system inertia and delay in automotive transmissions, feedback control of various components in automotive transmissions is not appropriate for certain transient elements. Control of transmission turbine speed during a kickdown shift is one example of a transient condition in automotive transmissions. During a kickdown shift, such as a drop from 4th gear to 3rd gear, or from 3rd gear to 2nd gear, the speed of the turbine must increase to correspond to a targeted gear ratio. Additionally, the acceleration of the turbine must be controlled to correspond to a targeted acceleration according to current vehicle acceleration. In such transient cases, feedforward control can be used to anticipate system changes. For example, mixed feedforward and feedback control can be used for a smooth kickdown shift without causing significant “feel” issues for the driver, thereby improving overall shift quality. Shift quality has been shown to be an important factor for driver satisfaction.
Automotive transmissions may use accumulators to absorb apply pressure fluid during certain shift operations. The presence of the accumulator reduces sensitivity of torque variations in torque phase during shifts. However, accumulators cause the pressure response to be slower and more difficult to predict since the solenoid current directly controls the flow rate and indirectly controls the pressure. With reference to FIG. 1, a typical accumulator 10 includes one or more springs 12 and a piston 14. Fluid fills the accumulator 10 and compresses the spring 12. The volume of the accumulator 10 varies over the usable range of the spring 12, and is indicative of the volume of a particular shift element in the transmission. The volume of the shift element is a further indicator of the capacity of the shift element, which may be used for control purposes. Target volume kickdown logic determines a target volume for the shift element, and subsequently calculates a change in shift element volume required for proper control. Control based on target volume can be used to calculate changes in element capacity or volume that are required to achieve target acceleration. As a result, excessive runaway or harshness during shifting is prevented.
Target volume control can be determined according to desired volume change due to turbine inertia force and/or desired volume change due to engine inertia force. Conventionally, empirical methods are used to determine target volume control. For example, change in volume can be calculated according to relationships between turbine inertia force, engine inertia force, accumulator pressure, and/or release element clutch pressure. However, such empirical methods are not particularly accurate in practice because turbine acceleration and engine acceleration each belong to independent dynamic systems. Therefore, the release element clutch cannot directly control engine acceleration. When the release element clutch is used to control turbine acceleration, turbine torque from the engine must be assumed as a fixed input through the torque converter and is a function of slip speed between the engine and the turbine.
If only the engine dynamic system is considered, the engine resistance torque, or turbine torque, can be changed to control engine acceleration if the throttle opening is fixed. However, turbine torque, or engine resistance torque, that is required to control the engine acceleration into a desired acceleration is different from the fixed turbine torque when turbine acceleration is controlled into a desired value. The control may be overcompensated because the torque required to change the engine acceleration is much larger than the turbine torque received from the engine. Therefore, it is desirable to provide optimized control during a kickdown shift to further improve shift quality. A continuous variable and speed-based desired acceleration method to provide consistent and accurate transmission control based in part on accumulator pressure is proposed.